a. FIELD OF THE INVENTION
This invention relates to heat transfer mechanisms and more particularly to heat transfer mechanisms for removing heat generated in electronic circuit modules.
b. RELATED ART
The use of very large scale integration (VLSI) technology in chip design has necessitated the development of special cooling techniques to accommodate the high heat fluxes generated by modern integrated circuit chips. One of the more attractive methods for removing heat from these chips is through the use of an immersion cooling system. In this system, the integrated circuits are immersed directly in a conventional dielectric cooling liquid. The liquid boils at the surface of the chips, and/or flows over the chips by natural or forced convection, thereby cooling the integrated circuits. The heat is then removed from the liquid through a remote heat exchanger.
A conventional manner of exchanging the performance of immersion cooling systems is the application of extended surface heat sinks. An example of an immersion cooling type electronic module using extended surfaces is disclosed in U.S. Pat. No. 4,203,129 to Oktay et al. and assigned to the same assignee as the present invention.
In the above-referenced module, semiconductor chips are mounted on a substrate by means of metal contacts or solder balls (also known as C4s). An extended surface heat sink, which is made of a thermally conducting material, and contains tunnels that run through the heat sink and lie parallel to the chip, is bonded on the back of each semiconductor chip. The module and tunnels are oriented vertically so that the tunnels act as chimneys to promote heat dissipation by natural convection and/or nucleate boiling. In addition, spring loaded heat-conducting pistons contact the back of the heat sink and provide conventional conduction cooling of the semiconductor chips. The pistons are seated in the module housing, and are cooled by an attached cold plate, which has a cooling fluid circulating therethrough. The module housing is filled with a dielectric fluid up to a level that does not quite fill the housing. The dielectric fluid cools the tunneled heat sink directly, and acts to improve heat transfer between the semiconductor chips and the pistons.
While the application of extended surfaces to enhance immersion cooling of microelectronic circuits can be advantageous, limitations of these surfaces are (1) the need to make compact cooling structures that will permit continued reductions in the chip-to-chip spacing in multi-chip arrays, and (2) the need to limit the height of cooling structures, especially in a 3 dimensional package where increasing the height of cooling hardware will increase the vertical module-to-module spacing. Consequently, there are limits on both the lateral and vertical dimensions of extended surfaces, and the associated added cooling area. In addition, the design and installation of a heat sink must be such that the benefit of added cooling surface area exceeds the resistance to heat flow into and through the heat sink.
Another effective method of removing heat from VLSI type integrated circuit chips is by direct jet impingement cooling. An apparatus employing direct jet impingement cooling is described in an IBM Technical Disclosure Bulletin article entitled "MULTI-ORIFICE FLUOROCARBON IMPINGEMENT COOLING FOR ARRAYS OF MICROELECTRONIC DEVICES " (TDB Vol. 32, No. 9A, February 1990, pp. 106-107). In direct jet impingement systems, a jet of liquid dielectric coolant is directed against the surface of the chip. Because there is no heat sink protruding beyond the edges or above the chip, advantages of direct jet impingement cooling are a compact module and no chip pitch limitations. However, if there were an accidental loss of coolant flowing into the module, chips that are not contacted by a solid heat sink can quickly heat to damaging temperatures. Also, although high heat fluxes can be attained by direct impingement cooling, the required coolant flow rate may be prohibitive. Cooling by jet impingement takes place primarily in a thin layer of fluid adjacent to the chip (the thermal boundary layer). High impingement cooling rates generally require a high velocity flow, in which case a great deal of fluid can bypass each chip without aiding in thermal dissipation. Bulk mixing of the fluid and increased area for heat transfer can be achieved with an extended surface, permitting similar cooling performance at lower flow rates, or improved performance at the same flow rate.
U.S. Pat. No. 4,964,458 to Flint et al. and assigned to the same assignee as the present invention, discloses the use of a flexible, finned sheet as a heat sink in a conduction cooled electronic module. The use of such a flexible sheet provides an advantage in that such a heat sink can efficiently utilize the available space in the module both laterally (in that the dimensions of the sheet are commensurate with the lateral dimensions of the module) and vertically (by the provision of suitable fins at the cooled side of the heat sink).
While the apparatus of Flint at al. provides for enhanced air/liquid conduction cooling, the above-referenced patent does not address the issue of applying the flexible sheet technology to an immersion or liquid jet impingement cooling environment. Furthermore, this sheet acts as a barrier between the chip-substrate structure and the coolant. As the sheet is made thin to insure compliance to chip height variations and chip tilt, the danger of corrosion or mechanical stress related failure of the barrier increases. (Compliance to chip topology is desirable to achieve a low resistance to heat flowing across the chip-to-sheet interface.) Water is superior as a coolant to known dielectric fluids, and the use of water may compensate for a high chip-to-sheet resistance to heat flow. Water, however, is electrically conductive and would cause catastrophic damage to the circuits if the barrier were to fail. Because water is a solvent and corrosive, it may actually precipitate failure of the barrier. In addition, the pressure on the chip and/or coolant side of the sheet must be regulated against sudden surges or reductions in the flow since, if the chip-side pressure ever exceeds the coolant pressure, the sheet could separate from the chips, causing a catastrophic loss of cooling.
In light of the above, there is a need for a heat sink, compact in both height and perimeter, that is suitable for use in an immersion and jet impingement cooled electronic modules.